1. Field of the Invention
The present invention generally relates to a solid-state image pickup device that removes fixed pattern noise from image pickup signals.
Priority is claimed on Japanese Patent Application No. 2008-313052, filed Dec. 9, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
In the field of area image sensor technology, further increase in the number of pixels and the speed of image pickup is necessary. Processing of signals needs to be performed at a higher speed. Processing of analog signals output from pixels in a circuit needs to be performed at a higher speed. Various methods of parallel reading out processing techniques such as reading out signals from pixels in a column at a time and transferring signals in a horizontal direction to a plurality of amplifiers in parallel are developed to achieve further increase in the number of pixels and speed in image pickup.
In parallel processing circuits that use the parallel reading out processing techniques, dispersion in a production process or the like causes dispersion in the characteristics of circuits. The dispersion in the characteristics of circuits generates fixed pattern noise including a striped defect on an output image.
The fixed pattern noise can be removed if noise pattern of the fixed pattern noise is recognized and stored. In a method, the noise pattern is stored in a line memory and is removed by digital signal processing.
When signals of pixels in a column are read out at a time in parallel, variations in characteristics of circuits that read out signals of pixels in a column cause variations in output signals. Variations in input-output characteristics of each column can be classified into roughly three variations, i.e., variations in offset properties, variations in gain properties, and variations in nonlinearity, depending on triggers of the variations. It is required that the variations in offset properties should be suppressed first among these variations, for the variations in offset properties are highly visible in an output image and digital correction for the variations in offset properties is easy.
To make the variations in offset properties recognized as a digital value, it is necessary to prepare a period that gives an input signal of the same fixed level to each of parallel circuits instead of a period that processes an effective pixel and to store variations of outputs of the circuits. The fixed pattern noise including striped defect having offset properties can be corrected by subtracting the amount of the stored variations from the image pickup signal of the effective pixel.
However, since output signals usually have random noises peculiar to analog circuits such as thermal noise and 1/f noise, a filtering process that reduces the random noises is necessary. Japanese Unexamined Patent Application, First Publication, No. 2005-167918 discloses a solid-state image pickup device that reduces the random noise using an averaging method, a correction value obtained by averaging correction values of a plurality of frames to improve the accuracy of the correction value.